In positron emission tomography (PET) imaging, a radiopharmaceutical agent is administered, via injection, inhalation, and/or ingestion, to a patient. The physical and bio-molecular properties of the agent then concentrate at specific locations in the human body. The actual spatial distribution, intensity of the point and/or region of accumulation, as well as the kinetics of the process from administration and capture to eventual elimination, all have clinical significance. During this process, the positron emitter attached to the radiopharmaceutical agent emits positrons according to the physical properties of the isotope, such as half-life, branching ratio, etc.
Each positron interacts with an electron of the object, is annihilated and produces two gamma rays at 511 keV, which travel at substantially 180 degrees apart. The two gamma rays then cause a scintillation event at a scintillation crystal of the PET detector, which detects the gamma rays thereby. By detecting these two gamma rays, and drawing a line between their locations or “line-of-response,” the likely location of the original annihilation is determined. While this process only identifies one line of possible interaction, accumulating a large number of these lines, and through a tomographic reconstruction process, the original distribution is estimated with useful accuracy. In addition to the location of the two scintillation events, if accurate timing—within a few hundred picoseconds—is available, time-of-flight calculations are also made in order to add more information regarding the likely position of the annihilation event along the line. A specific characteristic of the isotope (for example, energy of the positron) contributes (via positron range and co-linearity of the two gamma rays) to the determination of the spatial resolution for a specific radiopharmaceutical agent.
The above process is repeated for a large number of annihilation events. While every case needs to be analyzed to determine how many scintillation events are required to support the desired imaging tasks, conventionally, a typical 100 cm long FDG (fluoro-deoxyglucose) study accumulates about 100 million counts or events.
Conventionally, as shown in FIG. 9, detection of an event 900 is performed by a radiation detector, photomultiplier tube (PMT) 902. The PMT 902 has an analog output signal which is filtered by a filter 904 and converted from an analog signal to a digital signal by an analog-to-digital converter (ADC) 906. The filtered and converted signal is then output to a digital signal processing unit 908, which performs event counting and time sampling, executing algorithms for energy, timing, and positions of events. The filter 904 is conventionally designed to operate effectively at a variety of count rates, and conventionally includes an RC filter, as shown in FIG. 10.